# The fill factor of a solar cell

patric44
Homework Statement:
a question about the fill factor of a solar cell
Relevant Equations:
FF = Vm*Im/Voc*Isc
hi guys
for a solar cell i know that the fill factor is given by the following relation :
$$FF = \frac{Vm*Im}{Voc*Isc}$$
but when solving the following example :

the book used the following relation in the problem?! :

where this Ln relation came from ?